In a skillet set over moderately low heat melt butter, add leek, chard stems, and salt and pepper to taste. Cut a round piece of waxed paper to fit the size of your skillet and place it over the vegetables. Cover the skillet with a lid and cook for 5 minutes. Remove the cover and waxed paper. Now add the chard leaves, garlic, red pepper flakes, and broth and simmer the mixture stirring occasionally, uncovered for 5 minutes more. Increase the heat to high and reduce liquid until almost evaporated. Add the cream and walnut oil and simmer sauce over moderately high heat, stirring, until lightly thickened. Season with salt and pepper. Transfer cooked pasta to a large bowl. Add the sauce, toasted walnuts and toss to combine. Transfer pasta to serving plates and sprinkle with tarragon or parsley. Serve with freshly grated Parmesan cheese.